Hydraulic loading cranes are already known comprising a hydraulically rotatable vertical post to which is pivotally mounted an inner hoisting boom arranged for movement in the vertical plane, such movement effected by hydraulic means, and at the outer end of which boom an outer boom, a so-called jib is mounted so as to be hydraulically pivotable in a vertical plane. The jib is generally equipped with an extension boom which is displaceable in the longitudinal direction of the jib by hydraulic means and the outer end of which supports the load to be lifted.
It is also previously known to equip a hydraulically operated loading crane of this type with a valve system making it possible to perform all the movements of the crane through manual operation of the spools of the valve.
It is furthermore known to arrange the crane valve system in such a way as to make it responsive to electrical signals emitted from a control box to the so-called directional valve, the latter being equipped for this purpose with an electro-hydraulic converter, i.e. a so-called electro-hydraulic servo valve. This arrangement provides considerable advantages as it makes possible to give the control box convenient dimensions and configuration while at the same time it gives a freer choice of operating positions since the control signals may be transferred via a flexible and, if required, long electric operating cable.